Every site has a disclaimer/info/about page where you can read about license details for every image. In some site there is creative commons icon to indicate license details.        

Click here to learn more about CC license

To remove login form you must execute my software like administrator.

Follow these steps:

•Click on my software icon

•Right mouse click

•Click on “Run as administrator”

Now you can run my software without login form

The sites can change according what you are searching. To have the maximum choice you can use search engines like Google, Yahoo, Bing, etc. If you want totally free images (CC0 license) I suggest you to use pixabay.com